    public class Text4 {
    // 牛客 合并两个排序的链表
    public ListNode Merge (ListNode pHead1, ListNode pHead2) {
        // write code here
        if(pHead1 == null || pHead2 == null){
            return pHead1 != null ? pHead1 : pHead2;
        }
        // 两个链表元素依次对比
        if(pHead1.val <= pHead2.val){
            // 递归计算 list1.next, list2
            pHead1.next = Merge(pHead1.next, pHead2);
            return pHead1;
        }else{
            // 递归计算 list1, list2.next
            pHead2.next = Merge(pHead1, pHead2.next);
            return pHead2;
        }
    }
}
